1. Field of the Invention
The present invention relates to hinge devices, and more particularly to a spring hinge device which is provides balanced friction torque resistance when the hinge device is rotated in either of two opposite directions.
2. Related Art
A conventional hinge device for electronic equipment such as notebook computer comprises a helical spring fictionally wrapped around a shaft. The hinge device utilizes friction torque to enable smooth operation thereof. In a relaxed state, the spring has an inner diameter slightly less than the outer diameter of the shaft. The spring is thus frictionally engaged with the shaft so that the spring and shaft tend to rotate together. In use, the shaft in connected to a rotatable body of the notebook computer, such as a display thereof. When the rotatable body and the shaft are rotated, the spring being frictionally engaged to the shaft, tends to rotate accordingly. However, an end (or “toe”) of the spring is engaged in a stationary support. The engagement of the toe in the stationary support causes the spring to slip relative to the shaft rather than rotate with the shaft, and this generates resistance to rotation known as friction torque.
This kind of hinge device is disclosed in U.S. Pat. No. 5,503,491, which is incorporated herein by reference. In U.S. Pat. No. 5,503,491, a friction results from a unitary spring urging against the inner periphery of a tapering rod portion of a first shaft and the periphery of a rod portion of a second shaft. When the shaft rotates in a first direction (e.g. clockwise), the unitary spring winds in a right hand direction and tightens. While the shaft rotates in an opposite second direction (e.g. anti-clock-wise), the unitary spring winds in a left hand direction and loosens. Thus the hinge device provides different torque resistances in different directions of rotation. This results in the unitary spring applying an uneven force on the shaft, whereby the hinge device does not operate smoothly.
A hinge device which solves the above-mentioned problems is desired.